The Obama Administration used the State Department to halt the part of the pipeline that crosses the border into Canada. To me it seems like they couldn't stop the rest of the pipeline so decided with gas prices on the rise 'if you can't beat em join em.'
I don't think they ever had any interest in stopping the entire pipeline, just the portion that was an environmental threat. Allowing the rest of the pipeline to go through may look good politically, but it is not in any way a change of policy.